1. Field of the Invention
This invention relates to a novel precured pressure sensitive adhesive adapted for high temperature performance and having high tack.
2. Description of the Related Art
Pressure sensitive adhesives are materials which have tack properties for adhesion to a variety of dissimilar surfaces. Conventional pressure sensitive adhesive composition include elastomers such as natural rubber or synthetic polyisoprene, butadiene-styrene (SBR) polymers, thermoplastic rubbers or the like. The elastomers are combined with tackifying resins, fillers, antioxidants, etc. Natural rubber systems have high tack properties at room temperature. However, uncured rubbers generally do not perform satisfactory at high temperatures. At high temperatures, the adhesive looses its internal strength and leaves a residue of the adhesive on the application surface after the adhesive has been removed.
Conventional solutions for high temperature applications have included precuring or prevulcanizing the adhesive in situ after it has been coated on the tape backing. U.S. Pat. No. 2,999,769 describes curing a natural rubber pressure sensitive adhesive for providing resistance to adhesive deposit at elevated temperatures. U.S. Pat. No. 2,999,769 has the disadvantage that the adhesive is very sensitive to the state of cure. When the adhesive is undercured, the adhesive splits resulting in a deposit on the applied surface. When the adhesive is overcured, the hold at high temperatures is not satisfactory.
U.S. Pat. No. 3,625,752 (the '752 patent), issued to the inventor of this disclosure, describes a precured thermoplastic polymer which is relatively insensitive to the exact state of the cure achieved. The thermoplastic polymer has the structure A-B-A in which A is a thermoplastic polymer block of a vinyl arene and B is an elastomeric block of isoprene. Kraton 107 sold by the Shell Oil Company was used in the examples of the '752 patent as the thermoplastic polymer. Kraton 107 is frequently referred to in the trade as a 85% coupled polymer indicating that the composition is formed by 85% of A-B-A triblock copolymer and 15% of A-B diblock copolymer. It has been found that the use of the precured Kraton 107 polymer in an adhesive has superior high temperature holding properties over a wide range of curing conditions. However, the polymer has the disadvantage of having marginal tack properties in comparison to a cured natural rubber based pressure sensitive adhesive.
U.S. Pat. No. 4,080,348, issued to the inventor of this disclosure, describes an uncured adhesive including a thermoplastic rubber consisting of 55-85 parts of A-B block copolymer and 15-45 parts of a linear or radial A-B-A copolymer and a resin component. The A blocks are styrene and the B blocks are isoprene. The U.S. Pat. No. 4,080,348 has the disadvantage that it is an uncured adhesive having poor performance at high temperatures.
Of possible relevance to the invention are U.S. Pat. Nos. 3,923,722; 4,699,938; and 4,717,749, related to pressure sensitive adhesives.